diff --git a/scripts/errorCheck.ts b/scripts/errorCheck.ts
new file mode 100644
index 00000000000..77892cb8f0c
--- /dev/null
+++ b/scripts/errorCheck.ts
@@ -0,0 +1,87 @@
+declare var require: any;
+let fs = require('fs');
+let async = require('async');
+let glob = require('glob');
+
+fs.readFile('src/compiler/diagnosticMessages.json', 'utf-8', (err, data) => {
+	if (err) {
+		throw err;
+	}
+	
+	let messages = JSON.parse(data);
+	let keys = Object.keys(messages);
+	console.log('Loaded ' + keys.length + ' errors');
+
+	for (let k of keys) {
+		messages[k]['seen'] = false;
+	}
+
+	let errRegex = /\(\d+,\d+\): error TS([^:]+):/g;
+
+	let baseDir = 'tests/baselines/reference/';
+	fs.readdir(baseDir, (err, files) => {
+		files = files.filter(f => f.indexOf('.errors.txt') > 0);
+		let tasks: Array<(callback: () => void) => void> = [];
+		files.forEach(f => tasks.push(done => {
+			fs.readFile(baseDir + f, 'utf-8', (err, baseline) => {
+				if (err) throw err;
+
+				let g: string[];
+				while (g = errRegex.exec(baseline)) {
+					var errCode = +g[1];
+					let msg = keys.filter(k => messages[k].code === errCode)[0];
+					messages[msg]['seen'] = true;
+				}
+
+				done();
+			});
+		}));
+
+		async.parallelLimit(tasks, 25, done => {
+			console.log('== List of errors not present in baselines ==');
+			let count = 0;
+			for (let k of keys) {
+				if (messages[k]['seen'] !== true) {
+					console.log(k);
+					count++;
+				}
+			}
+			console.log(count + ' of ' + keys.length + ' errors are not in baselines');
+		});
+	});
+});
+
+fs.readFile('src/compiler/diagnosticInformationMap.generated.ts', 'utf-8', (err, data) => {
+	let errorRegexp = /\s(\w+): \{ code/g;
+	let errorNames: string[] = [];
+	let errMatch: string[];
+	while (errMatch = errorRegexp.exec(data)) {
+		errorNames.push(errMatch[1]);
+	}
+
+	let allSrc: string = '';
+	glob('./src/**/*.ts', {}, (err, files) => {
+		console.log('Reading ' + files.length + ' source files');
+		for(let file of files) {
+			if (file.indexOf('diagnosticInformationMap.generated.ts') > 0) {
+				continue;
+			}
+
+			let src = fs.readFileSync(file, 'utf-8');
+			allSrc = allSrc + src;
+		}
+
+		console.log('Consumed ' + allSrc.length + ' characters of source');
+
+		let count = 0;
+		console.log('== List of errors not used in source ==')
+		for(let errName of errorNames) {
+			if (allSrc.indexOf(errName) < 0) {
+				console.log(errName);
+				count++;
+			}
+		}
+		console.log(count + ' of ' + errorNames.length + ' errors are not used in source');
+	});
+});
+
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 866d7d9b261..b43345bcbf5 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -2867,7 +2867,7 @@ namespace ts {
                     });
 
                     if (isArrowFunctionInJsx) {
-                        return Tristate.Unknown;
+                        return Tristate.True;
                     }
                     
                     return Tristate.False;